(a) Field of the Invention:
The present invention relates to a compact zoom lens system having a zooming ratio of no less than 2 wherein aberrations are favorably corrected.
(b) Description of the prior art:
Recently, together with the progress in the compactness of cameras, there has been required a compact zoom lens system having a short length. In lens shutter cameras, in which automated systems have been developed and ease of carriage has been regarded as important, the limitation of a back focal length is not so severe but the requirement of more compactness is strong.
As a lens system of this kind, a zoom lens system for use in a lens shutter camera characterized by a simple lens configuration has been disclosed in Japanese Published Unexamined patent application No. 201213/82. This zoom lens system is of so-called two groups-type which comprises, in order from the object side, a first lens group having a positive refracting power and a second lens group having a negative refracting power and is arranged to be zoomed by varying the space between these lens groups.
In Japanese Published Unexamined Patent Application No. 137813/83, moreover, there has been disclosed a zoom lens system of three groups-type which is considered to be developed from the above-mentioned two groups-type zooming system. In a zoom lens system having such a configuration, in the case where a zooming ratio thereof is about 1.5, it is possible to make a compact lens system which comprises a small number of lens components. However, in the case where the zooming ratio is made to be about 2, magnification shared by a lens group used as a variator will become large so that the fluctuation of aberrations during the zooming operation will become large and the number of lens components necessary for the correction of aberrations will increase, which results in the enlargement of the zoom lens system as a whole. Therefore, for a zoom lens system having such a lens configuration, it is impossible to make the optical performance favorable and to attain the compactness of the zoom lens system.
In order to overcome the problems mentioned in the above, a zoom lens system of two groups-type has recently been developed which comprises a graded refractive index lens, i.e., a lens made of the medium whose refractive index varies gradually according to the radial distance from the optical axis. This system has been disclosed in applications such as in Japanese Published Unexamined patent application No. 148414/86, etc.
This zoom lens system comprises, as a lens component, a graded refractive index lens of a radial type which has the distribution of refractive index in the radial direction from the optical axis so that the refracting power is shared by the lens medium to thereby control the fluctuation of aberrations during the zooming operation and favorably keep Petzval's sum which contributes to the flatness of an image surface while maintaining the refracting powers of the respective lens components to be strong.
In this zoom lens system, the object thereof is to have a zooming ratio up to 2 and the correction for Petzval's sum is attained sufficiently. However, other aberrations are not corrected sufficiently, and this remains a problem to be solved hereinafter. Moreover, the zoom lens system has the defect that the back focal length thereof was extremely short so that the problem has not been solved in that the rear lens group thereof becomes large in size and the compactness thereof is not sufficiently attained.